1. Field of the Invention
This invention relates to a dock leveler which acts as a bridge between a loading dock platform and the bed of a carrier and, more particularly, to a pit-mounted hydraulic dock leveler.
2. Description of the Related Art
Dock levelers for bridging a gap between a loading dock platform and a carrier bed are in widespread use to facilitate the loading and unloading of various types of carriers. Forklift trucks are often required to move from a loading dock directly into and out of a storage compartment of a carrier. In order to facilitate such an operation, dock levelers are used to compensate for any height variation which exists between a loading dock platform and the bed of a carrier.
Typically, a dock leveler will have a main deck which pivots relative to an adjacent loading dock platform and a lip member which pivots relative to the main deck. The lip member will rest directly on the bed of a carrier when the dock leveler is positioned as a bridge and will support the main deck when the dock leveler is in the stored position. Movement of the main deck and lip member can be achieved by either mechanical means or by fluid pressure. It is a hydraulic dock leveler operating by fluid pressure to which the present invention is particularly directed.
There are two basic operational modes of a dock leveler. The first of these modes is commonly referred to as cycling. During the cycling mode, the dock leveler moves from a stored position to a bridging position for bridging the gap between a loading dock platform and the bed of a nearby carrier. The preferable stored position for a dock leveler is with the main deck extending substantially level or co-planar with an adjacent loading dock platform so that the dock leveler is in effect, an extension of the loading dock platform. When the dock leveler is in a bridging position, the lip member rests on and overlaps the bed of the carrier so that a forklift may move directly from a loading dock platform across the main deck and over the lip member directly into the storage compartment of a carrier.
The second operational mode of a dock leveler is commonly referred to as recycling. During the recycling mode, the dock leveler moves from the bridging position to the stored position.
The speed with which a dock leveler cycles and recycles is a critical factor affecting the volume of material which can be loaded or unloaded from a loading dock during any given time period. In a busy environment such as a loading dock, increased speed in recycling and cycling a dock leveler directly translates into increased profitability for a company. Accordingly, dock levelers with higher speeds of operation are constantly in demand.
Aside from relatively low speeds of operation, a second problem encountered in prior art hydraulic dock levelers involves pivoting the lip member from the bridging position to the stored position to support the main deck at a selective level relative to the loading dock platform. The generally accepted method of moving the lip member from the bridging position to the stored position is to drain the hydraulic fluid from the lip cylinder while the lip member is resting on the carrier bed. The lip member will then drop by gravity to its stored position after the carrier on which the lip member is resting departs. Such a system is disclosed in U.S. Pat. No. 3,345,665. However, many operational problems are created by relying on gravity to move a lip member to the stored position.
Typically, a lip member in a stored position will form an angle less than 90.degree. with the main deck. In other words, the lip member pivots past a perpendicular plane relative to the main deck to reach its stored position and engage keepers provided on the frame of the dock leveler. Relying solely on gravity to move the lip member past a perpendicular plane relative to the main deck creates a serious operational problem because there is no force to hold the lip member in its stored position. Often, the lip member will swing downward under the force of gravity to its stored position and then swing outwardly to a position were it will not engage the keepers.
A second problem associated with relying on gravity to move the lip member to the stored position occurs when the hinge which pivotally connects the lip member to the main deck is damaged. Such damage can occur from trucks backing into the hinge while maneuvering close to the loading dock or can result from unfavorable climatic conditions since the hinge is susceptible to outside weather conditions. A damaged hinge will slow the descent of the lip member to its stored position. Further, a damaged hinge may prevent a lip member pivoting solely under the power of gravity from reaching its stored position.